Need a larger list of names, can't think of any off hand.
some ideas:
1. Can Hero be evil or just pretending...?Once Hero gains respect within rebel faction, he is expected to act as an Infiltrator, sabotage Overlord facilites, gather resources and information. This can only be pulled off if Hero show's the same level of merciless cruelity as expected of any worthy overlord subordinate or atleast express a little of it. Is a demonstration of loyality necessary to pull off the Infiltration?
2. More about father & overlord.... Cities claimed by Big Brother more high tech, but still mostly powered by steam...
Hero makes another discovery about his father during infiltration of cities claimed by overlord.
His suspicions are raised when he realises there is less steam technology around in these then he remembered. Theres more advanced stuff everywhere (including law enforcement mechanoids / robot guards, etc...) and stuff that he remember's his father mentioned about his work only once (rare enough for him to remember).
Hero discovers more when he makes contact with a former scientist associate of his father, a rebellion sympathiser who did not have the courage of his father (who the Overlord promoted for assisting him in realising Hero father's technological blueprints when hero family was split up). His father was in the Overlords bad books long before the family split up. His father had discovered a novel energy power source and invented many new non-steam based technologies that were never finished. Overlord saw this technology as a key advantage in expanding his empire. The Overlord wanted ownership of these without giving any credit. When father refused it was an ideal opportunity for the overlord to get rid of him. Accusaton of hoarding technology that could benefit society was made. An act of betrayal against civilised society ( as the propaganda would go).
At a latter stage Hero witnesses same associate arrested for being a rebellion sympathiser and promptly executed without fanfare. A witch hunt for all rebellion sympathisers after several successful campaigns (of "terrorist act" = propaganda) against the regime gave the Overlord securities a green card to do as a they pleased. This only furthers the rebellions cause and support.
3. Eugenics and zombies...This group has defined a hierarchy of worthiness, where the lowest levels must die or be made slaves...
This could be expressed geographically too. The underground system could be a tiered system where people are allowed to only occupy locations deemed worthy of them. Briefly, if there was a worthiness scale, those considered useful would occupy a location closer to the surface and those less deeper.
Only a minority of the population live on surface, lesser sorts are expected to commute to surface in preparation for daily servitude. There is also a regular culling of the subterranean populace for slaves.
There could be those that are classed as not even being worthy of a worthiness scale, relegated to the lowest levels and ignored (out of sheer arrogance, a mistake that could be advantageous to the rebellion?). This ignored group could shift the story line in another direction.
Another unexpected scenario could be they are hunted down to the point where they have been forced to escape to the deepest uninhabited levels, living a wretched existence amongst the filth of waste repoduce that is dumped here via massive drainage piping / infrastructure. The darker side could be that some of these were criminals who were experimented on and disposed of here, in the raw sewage, industrial waste, rotten food, radioactive sources they mutated, sort of zombie like creatures? On a gaming front, the rebellion discovered this when they ventured here to replenish there resources (various bio-gases, poisons, acids, radioactive sources, other dumped materials). Our hero would discover this from the rebellion when he is put on material resources gathering, recruitment qualification test? How much can he collect and still return alive, with a few zombie souvenirs? Oh, to make a real drama out of this, one of the zombie's could be his brotther/sister. When family was split up, he had no idea where all the various relations ended up. He slowly discovers all their whereabouts during his tasks and infiltration work for the rebellion. As an act of mercy he decides to swiftly despatch his zombie sibling ( at a character level this could be a personailty building experience, more hardened resolve? more disdain for the regime, more prepared to dare an audacious revenge?).
[EDIT] cleared up typos, I've got a new rubbishy wireless keyboard that has sticky rubbery keys, damn annoying, lol.